1. Field of the Invention
The field of the invention relates to a ground proximity warning system for an aircraft and, more particularly, to a ground proximity warning system and a detector that is able to sense various phases of flight, such as a landing phase of flight, for aircraft with fixed or non-retractable landing gear, such as a DeHavilland DHC-6 Twin Otter aircraft.
2. Description of the Prior Art
Ground proximity warning systems are generally known in the art. Examples of such systems are disclosed in U.S. Pat. Nos. 4,030,065; 4,792,799; 4,849,756; 4,939,513; 4,951,047; 4,987,413; 5,038,141; 5,187,478 and 5,220,322. Such ground proximity warning systems are used to provide a pilot of an aircraft of an indication of a dangerous flight condition, such as inadvertent contact with terrain. In order to minimize nuisance warnings, detectors are known that detect the phase of flight of the aircraft in order to modify the respective warning envelope during certain conditions. For example, a terrain clearance or Mode 4 warning system normally provides a warning when an aircraft is too close to the terrain when the aircraft is flying above a predetermined speed. An example of such a terrain clearance warning system for aircraft with retractable landing gear is disclosed in U.S. Pat. No. 4,030,065, assigned to the same assignee as the assignee of the present invention and incorporated herein by reference. In such a warning system, the warning envelope is adjusted to prevent nuisance warnings during a landing phase of flight. For retractable landing gear aircraft, the landing phase of flight is normally sensed by way of a discrete or position switch on the landing gear.
Because the landing gear position plays such an important role in ground proximity warning systems, the implementation of such systems on aircraft with non-retractable landing gear, such as the DeHavilland DHC-6 Twin Otter aircraft, have been less than ideal. In such an application, any ground proximity warning system would have to be configured for either a cruise phase of flight or a landing phase of flight. If such a system were normally configured for a cruise phase of flight, nuisance warnings would be generated every time that the aircraft enters a landing phase. Although configuring the warning system for a landing phase of flight would eliminate the nuisance warnings during a landing phase of flight, such a configuration would also eliminate much of the protection for the aircraft in a cruise phase of flight.
In order to resolve these problems, various other approaches are available. For example, one approach is to provide a switch in the cockpit which the pilot of the aircraft would manually set to correspond to the current phase of flight, for example, cruise or landing. This switch would be integrated into the ground proximity warning system in a similar manner as the landing gear discretes are incorporated into the warning systems for aircraft with retractable landing gear. The problem with such an approach is that it is totally dependent upon the diligence of the pilot. Should the pilot forget to set the switch to the current phase of flight, the ground proximity warning systems would either generate nuisance warnings during a landing condition in the event that the switch was left in a cruise phase of flight or, in the event that the switch was left in the landing phase position, provide less than ideal protection during a cruise phase of flight.
Other approaches for detecting the phase of flight of an aircraft with non-retractable landing gear include sensing the airspeed, radio altitude or throttle position. Due to the relatively wide dead band of known airspeed sensors and the relatively low airspeed at which such fixed gear aircraft operate, coupled with the relatively wide range of airspeeds on approach used by such aircraft to enhance separation distance, measurement of airspeed as an indication of a landing phase of flight would provide unacceptable results. Sensing radio altitude would likewise provide unacceptable results because in certain situations, for example, level flight over rising terrain, would falsely indicate a landing phase of flight. Lastly, due to the variance in power settings and, consequently, throttle positions during a landing phase of flight, throttle position is also an unsuitable parameter for detecting a landing phase of flight because of speed variations and, consequently, throttle position variations resulting from the need to provide spacing from other aircraft; the need for additional power if the approach altitude is too low; or a condition when aircraft is in a sinking mass of air.